Manufacture of collagen products



United States Patent ()fiice 3,071,483 Patented Jan. 1, 1963 3 071,483MANUFACTURE 01 COLLAGEN PRODUCTS Shu Tong Tu, Ipswich, Mass, assignor toUnited Shoe Machinery Corporation, Boston, Mass, a corporation of NewJersey No Drawing. Filed May 3, 1960, Ser. No. 26,399 7 Claims. (Cl.106-155) This invention relates to the preparation of tanned collagenfiber masses and particularly to the preparation of a leather-like sheetmaterial.

In the United States Letters Patent 2,934,446 of April 26, 1960, to JohnH. Highberger and Robert A. Whitmore, entitled Collagen Fiber Masses andMethods of Making the Same, there is disclosed a method for forminginterlocked collagen fiber masses. That method involves forming amixture of existing collagen fiber and collagen in aqueous solution at acontrolled ionic concentration and pH, and maintaining the mixture at atemperature below about C. to prevent precipitation from the collagensolution. The cold mixture is shaped as desired, for example intosheets. Thereafter, by warming, the collagen fibers are precipitatedfrom solution in interlocking relation with the existing collagen fibersto form a skin-like material. This skin-like material is capable ofbeing tanned to a leather-like condition either with or withoutintermediate treatments.

Operation under the process of the above-identified application involvesthe problems of maintaining the mixture below 5 C. during shaping, andof warming the shaped mixture under controlled conditions. The productobtained after precipitation by Warming is at a pH value of from about5.2 to 9.5 and it is necessary, in order to convert this to aleather-like condition, to immerse this material in an acid solution ofa tanning agent such as a chrome or vegetable tanning" agent.

It is an object of the present invention to form a collagen fiber massfrom dissolved collagen by a procedure which eliminates the necessityfor control of temperature.

It is a further object of the invention to accomplish in a single stepthe operations of converting dissolved collagen material mixed withexisting collagen fibers to a leather-like condition.

To this end, and in accordance with a feature of the present invention,I have devised a process for precipitating fibers of collagen fromsolution with an agent effective to combine with collagen and toprecipitate it from solution as fibers in interlocking relationship witheach other and with existing collagen fibers present in the collagensolution.

I have discovered that collagen may be precipitated from solution at pHvalues as low as about 3 by the action of chromium compounds reactivewith collagen. This precipitation is perfected by contacting thematerial containing dissolved collagen with an aqueous solutioncontaining the collagen-reactive chromium compound.

In solutions more acid than about pH 5.2, collagen molecules in solutionhave net positive charges which prevent their aggregation into fibersand precipitation. According to one form of the process of the presentinvention, compositions including such more acid solutions of collagenare prepared and shaped as desired, the acidity preventing prematureformation of collagen fibers even at room temperature or above.Thereafter, the acid compositions are treated with a metal compound,e.g. a chromium compound, eifective to precipitate collagen fiber in theacid media.

In a preferred form of the invention, acid collagen solution is mixedwith existing collagen fibers to form an acid mixture and this mixtureis treated with a soluble chromium compound to precipitate the collagen.The invention will first be described as it is practiced in this form.However, it is to be understood that the collagen solution need not bein acid form provided it is otherwise prevented from prematureprecipitation and provided also that a sufficient acid value isprovided, either in the solution of the chromium compound or in thecollagen solution, that the pH of the solution of metal compound doesnot rise above about 5.

Collagen solutions useful in the present process may be formed byprocedures known per se, such for example as the procedure shown in theapplication of John H. Highberger and Robert A. Whitmore referred toabove. The procedure may include the steps of roughly grinding youngmammalian skin, suitably calfskin, and dispersing the ground skinmaterial in an aqueous acid bath at a pH of from 2 to 4.5. Usable acidsinclude formic acid, acetic acid, propionic acid, citric acid,phosphoric acid, hydrochloric acid, sulfuric acid and other commonorganic and mineral acids which do not precipitate protein. The skinmaterial is allowed to stand at a temperature below 25 C. and preferablynear 0 C. for from 12 to 48 hours. The acid bath containing skinmaterial is then preferably passed through a mechanical device tosubject it to severe shearing action which brings the product to a pastycondition in which a high percentage of the collagen of the originalskin material appears to be in solution.

Hide fiber material for combination with collagen solution may beprepared by the procedure described in greater detail in the copendingapplication of Frederic C. Merriam et al., Serial No. 846,254, filedOct. 14, 1959, and entitled Preparation of Hide Fiber. As described morefully in that application, skin or hide material, such as limed unhairedhide, pickled hide, or unlimed hide, is Washed, subdivided into pieces,preferably not smaller than one inch, and preferably chemically treatedas by limited formaldehyde tannage to reduce the swelling tendency ofthe collagen fibers of the skin or hide. The treatment, which should besufficient to provide a minimum of at least V of combined formaldehydebased on the dried weight of the fibrous material and preferably notover 2%, brings the skin or hide material to a condition in which it canbe reduced most effectively to its separate fibers.

The hide material is placed in a Water bath and subjected to amechanical device involving relatively moving surfaces which exert ashearing action for pulling or teasing the hide material into itsconstituent fibers and forming a pulp or suspension of the fibers inwater.

The fibers and collagen solution may then be mixed in suitable mixingequipment. The collagen solution is acidic having a pH usually around3.5 and the fibers are somewhat more alkaline, so that the mixture has apH somewhat higher than desire The pH of the mixture may be reduced byaddition of acid to a value of about 3 to 4 and the mixture worked intoa sheet as by rolling or pressing a mass of the material to a desiredthickness.

Precipitation of collagen from the acid composition containing dissolvedcollagen, such as the sheet of the mixture of dissolved collagen andexisting collagen fibers, may be effected by contacting a compositioncontaining dissolved collagen with an aqueous solution of a chromiumcompound reactive with the collagen. The chromium compound precipitatesand combines with the collagen from solution and also with existingcollagen fiber in the composition. There is reason to believe that thechromium compound 'acts to establish cross linkages between collagenfibers, both those formed from solution and those already present in thecomposition before treatment. These cross linkages increase further thestrength of the mass of intertwined pre-existing and precipitatedcollagen fibers by anchoring the fibers against relative slippage.

.atemperature below Compounds effective in aqueous solution toprecipitate collagen from solution include the water soluble compoundsof trivalent or hexavalent chromium. In particular, the compounds formedby partially neutralizing chromic salts such as chromic chloride andchromic sulfate give very desirable results. The reaction of thesechromic salts with alkaline material brings the salt first to the stagesreferred to by tanners as one-third basic and then to higher basicity upto as high as about 50% basic with an optimum of for the presentpurpose. Use of the basic chromium solution not only precipitatescollagen from solution but also tans both precipitated and existingcollagen to give a desirable relatively high shrink temperature.

Precipitation of collagen from solution occurs in the pH range of about3.0 to about 5. The pH of the chrome solution and/or the materialcontaining dissolved collagen are adjusted so that when the solution andmaterial are brought together, the resulting pH is in this range.

A masking or complexing agent such as the ammonium and alkali metalsalts of formic acid and of hydroxy carboxylic acids including sodiumtartrate, sodium phthalate or sodium gluconate is used with the chromicsalts, these agents being capable of forming complex molecules with theproducts formed by partial neutralization of the salts. The complexcompounds thus formed are relatively stable so that the pH of thesolutions may be raised up to as high as about 5 without precipitationof chromic oxide. For use in the process of the present invention, thesolution is ordinarily adjusted by addition of an alkaline material suchas sodium hydroxide to a pH of from about 3 to about 5, preferably about4. It is to be noted that with the complexed solutions the pH of thesolution may require as much as two days to stabilize, during which timeadditional alkaline material may be added to achieve the desired finalpH value.

The chromium solutions should contain about 1% to 2% by weight of thecompound calculated on the basis of the weight of the oxide and maycontain as much as 5%. It has been found that stability of chromic saltsagainst precipitation is obtained where as little as one part by Weightof complexing agent is used for two parts by weight of the chromiccompound, and that there may be used as much as about three parts byweight of the complexing agent to one part of the chromic compound. Thepreferred compositions contain chromium, calculated as the oxide, andcomplexing agent in the ratio of approximately 1:2.

Compositions containing dissolved collagen may be either cold, e.g.below 5 C., or at room temperature when brought in contact with thesolution of the chromium compound. That is, with compositions which havea pH below 5.2, collagen fiber will not precipitate from the solution atroom temperature. Somewhat different results are obtained when thecomposition is at room temperature than when it is cold. It appears thatthe cold composition reacts somewhat less vigorously with the chromiumso that more uniform impregnation is obtainable and also the time fordevelopment of collagen fibers is somewhat greater so that the productis somewhat stronger than where precipitation is carried out with a roomtemperature composition.

It is also possible to combine the chromium precipitation of collagenwith the thermal precipitation of the Highberger et al. procedurereferred to above. Thus, a cold mixture of collagen solution andundissolved fiber at controlled ionic concentration and pH of 5.2 to 9.5and ionic strength of 0.1 to 1.0 may be shaped while at C. to preventprecipitation of collagen from solution and thereafter brought incontact with a solution of chromium compounds to precipitate collagenfrom the solution. Particularly where the cold mixture of collagensolution and fiber is formed into a sheet, it has been found thatwarming the sheet to effect partial thermal precipitation of thecollagen fiber Bauer mill.

before chromium treatment presents certain advantages. The partialprecipitation gives strength to the sheet so that it may be handled moreeasily without disruption, for example, in transferring it to apparatusfor treatment with chromium compound. There is some indication that thepreliminary partial precipitation of the collagen through warmingaffects the character of the sheet. That is, a sheet so prepared appearssomewhat more open and is somewhat more compressible than a sheetprepared without such warming.

The combination of chrome tanning agent with collagen provides as aknown characteristic the increase of the shrink temperature of thecollagen fiber. It appears that in the present relationship, where thechrome compound is a tanning material, the chromium acts on the collagenmaterial and the heat resistance-increasing effect of the chromiumexerts itself on the collagen components even before a full tanning.That is, full reaction of the chromium material with the collagenmaterial can be speeded up without detriment to the collagen by warm ingthe chromium bath to a temperature which may be as high as 50 C. Rapidand complete interaction is obtained with no observable degradation ofthe collagen even though heating a solution of collagen to so high atemperature would result in at least partial degradation of thecollagen.

The product of contacting material containing dissOlVed collagen withchromium solutions to precipitate collagen many he treated according toknown procedure. It may be dyed with water soluble acid or directdyestuffs. The product may be treated to incorporate a humectant, e.g.sorbitol, and/or a fat liquor such as an emulsion of meatsfoot oil.Where the product is in sheet form it is desirable to subject it,preferably after drying, to a needling operation such as that describedin my copending application Serial No. 805,032, filed April 8, 1959,entitled Collagen Fiber Sheet Material and Method of Making. Any finishapplicable to leather may be applied to the product.

The following examples are given as of possible assistance inunderstanding the invention; but it is to be understood that theinvention is not limited to the specific materials or conditions shownin the examples:

Example I 43 lb. of salt calfskin scraps were unhaired, washed, andadded to lbs. of water containing 54 cc. of a commercial H PO and 138cc. of acetic acid. After stirring the hide in, the pH was 3.1. The bathwas allowed to stand six days with occasional stirring. At the end ofthis time the scrap material was well swollen and the pH was 4.0. Thescraps were cooled with cold water, drained, and run several timesthrough a mill comprising closely spaced, relatively moving roughplates, i.e. a The milled product weighed about 100 lbs. To this wereadded 20 lbs. of water containing 42 cc. of mixed acid (138 cc. ofacetic acid and 54 cc. of phosphoric acid). After stirring and standingbriefly the pH of the mix was 3.8. The mix was then sent through theBauer mill again and 40 additional lbs. of water and 100 cc. of the sameacid mixture were added and stirred in. After standing overnight the pHof the mix was 3.7. 20 lbs. of water and 92 cc. of the same acid mixturewere added to the mix and the mix was then sent through the Bauer millusing close set, fine plates and came out as a smooth pasty mass at pH3.7. In this mass a high proportion of the collagen initially presentwas in solution.

Pickled split cowhide trimmings were washed and brought to a pH of 8.5by addition of sodium hydroxide. The trimmings were cut to approximately1 pieces and 50 lbs. of the trimmings were placed in a Hollander typepaper beater with lbs. of water to give a solids content of about 5%.The beater was operated for /2 hr. at the end of which time the pH wasreadjusted to 8.5 and 300 cc. of 37% formaldehyde were added. Theheating I solids concentration to about 25%.

was continued for 5 minutes and the resultant slurry was allowed tostand for 2 hrs., then discharged onto screens and drained overnight.The drained material was passed through rubber squeeze rolls to bringits The squeezed material was then put back in the beater and wateradded to bring the solids content in the beater to about 5%. The heaterwas operated for one hour with the beater knife close to the bed plate(0.003 to 0.005" clearance). The slurry was then removed from thebeater, drained overnight and squeezed through rubber rollers to give afibrous mass of about 26% solids.

To 2700 grams of the slurry from the Bauer mill at from to C. there wereslowly added in a Worm type mixer 2600 grams of the drained and squeezedfiber and 2700 cc. of water. This mixture had a pH 'of about 4.5. Aftermixing for 1 hr. the mixture was placed on a polyethylene sheet, coveredwith a sheet of polyethylene terephthalate resin (Mylar) and spread byrolling and working to a thickness of 0.1".

The resin sheet was stripped from the layer of mixture and the layer ofmixture was laid face down on a porous support, after which thepolyethylene sheet was stripped. The deposited sheet of mixture was thenimmersed in a chromium solution of the following composition:

Parts by weight The chromium solution was made by dissolving the chromicsulphate in the water and adding the sodium formate to the solution. ThepH of this solution was less than 4 and was adjusted to pH 4 by additionof sodium hydroxide. The resulting solution was allowed to stand for twodays to stabilize the addition of further sodium hydroxide to maintainthe pH at about 4. Analysis of the resultant chrome solution showed achromic oxide content of 1.3% and a sodium formate concentration of 2%.

The sheet of pre-existing collagen and collagen from solution wasmaintained in contact with the chrome solution for one hour and was thenremoved and allowed to stand overnight. The sheet was washed three timeswith tap water and solvent dried with acetone. It was found that thesheet had taken up about 2% of chromic oxide based on the weight of thecollagen material.

The sheet was soaked in a 4% solution of oleic acid in isopropyl alcoholfor one hour. The sheet was then withdrawn and allowed to dry. The driedsheet material resembled leather and on testing had a shrink temperatureof 100 C.

The sheet was then passed between a reciprocating bar carrying needleson its lower surface and the sheet manipulated beneath the needle bar toprovide an average of 1500 perforations in the sheet per square inch.The diameter of the holes was from about 0.1 to about 0.2 mm.

After the needling treatment a 20% solid solution of a butadieneacrylonitrile copolymer synthetic rubber latex Was spread on the surfaceof the sheet. The latex contained 5% of a wetting agent (Triton X-200)and was spread on in amount to provide 5% solids based on the weight ofthe dried sheet. The sheet was dried and thereafter conventional acrylicresin emulsion finish was spread on the surface of the sheet and allowedto dry.

Thereafter the sheet was finished by conventional procedure includingapplication of a commercial base coat containing pigment, wax and resin.The sheet was embossed by the usual commercial embossing procedure andfinally a coat of an aqueous dispersion of lacquer was applied as a topcoat and the sheet was plated. The resulting product resembled a goodgrade of natural leather.

Example II 120 lbs. of trimmings from green salted light (4-6 lbs.)calfskins were placed in a drum and washed in running cold water at adrum speed of about 12 r.p.m. for about 12 hours to remove the salt. Thetrimmings were then drained and passed through a plate-type meat grinderof which the plate had circular holes. The ground material was washedand drained, and the drained mass weighed about lbs.

To the washed and drained mass 2.5 lbs. of glacial acetic acid wereadded and vigorously stirred in. The acidified mass had a pH of about3.75 and a dry solids content of about 10%, and was allowed to stand atroom temperature for two days.

After standing the acidified skin material was cooled by mixture withchipped ice and passed through a platetype shearing mill of which theplates had an edge clearance of about 0.005" to 0.010". The materialleft the mill as a smooth plaste whitened by occlusion of fine airbubbles. On standing the air bubbles became aggregated, absorbed, orremoved, leaving an opaque, tan-colored paste having a solids content ofabout 7%.

Trimmings from lirned steer hides were washed in a rotating drum (12rpm.) for two hours in running tap water. lbs. of the washed trimmingswere placed in 20 gal. of water and 400 ml. of glacial acetic acid wereadded and thoroughly stirred in to partially neutralize the trimmings. Acut section of the trimmings showed a pH of about 4. The trimmings wereagain-washed with running water in a drum for a period of about 1 /2hours and thereafter were drained. An aqueous solution of 6.25 lbs. of awetting agent in 6.25 lbs. of water at 30 C. were added to the trimmingsand drummed in. Thereafter, water at 30 C. was introduced into the drumand washing was continued with drumming until the efliuent was clear.The trimmings were drained. Thereafter 30 gal. of water and 2850 ml. of37% aqueous formaldehyde solution were drummed into the trimmings for 30minutes. The trimmings were then removed, placed in a prepared mixtureof 20 gal. of water and 1900 ml. of 37% aqueous formaldehyde solutionand stirred to eifect a uniform mixture. The pH of the mixture wasbrought to about 8 by the addition of 54 grams of sodium hydroxide andthe material was allowed to stand for 5 days. The trimmings were thenwashed in the drum in running water for 2 hours. The material was cutinto 1" strips and then passed through a plate-type meat grinder havingholes. The ground material was drained and introduced into a Hollanderpaper beater in which the material was beaten for 10 to 15 minutes afterthe bumping, due to the presence of large lumps, had stopped. The beatenmaterial which was a uniform suspension of hide fibers was drained on a20 mesh screen. The mass of drained fibers was then passed betweentightly pressed rubber rolls to remove additional water. At this pointthe formaldehyde content of the fibers was about 1% on a dry solidsbasis; and the dry solids content was about 25% of the pressed fibermass.

2030 grams of the acidfied calfskin material were placed in a 1 gal.jar, and cooled by addition of about 1000 grams of chipped ice tobetween 0 and 5 C. A solution of 46.98 grams of K HPO in about200 ml. ofwater was added and mixed in thoroughly and the mixture was thenneutralized with about ml. of 6 N KOH to give a pH of about 7.5. Ice andwater were added to bring the volume of the mixture to 1 gal. and thefinal mixture to an ionic strength of about 0.4. 1333 grams of thischilled neutral mass was placed in a screw-type, jacketed mixer whichhad been precooled to from 0 to 2 C. by circulating a salt and ice brinethrough the jacket. The mixer was started and 1136 grams of the pressedfiber was introduced slowly in small bits torn from the mass of fibers.The material was pressed down in close ing for a period of about onehour until the mass was a smooth, uniform paste which was doughy in itsresistance to extreme deformation. The mixture was taken from the mixerand rolled into a sheet A" thick using rolling strokes, first in onedirection and then in a direction perpendicular to the first directionin order to move the fiber-containing mixture uniformly in alldirections in the plane of the sheet. The mixture was kept below C. fromthe time when it was first prepared until rolling was completed.

The sheet was then immersed in a chromium solution of the samecomposition as that used in Example I and maintained in contact with thesolution for one hour. The sheet was then removed from the solution andallowed to stand overnight, after which it was washed three times withtap water and solvent dried with acetone. It was found that the sheethad taken up about 2.5% of chromic oxide based on the Weight of thecollagen material.

The sheet was then treated by the procedure set forth for the treatmentof the chrome reacted sheet of Example I and formed a product resemblinga good grade of natural leather.

Example 111 A sheet of a mixture of hide fibers and the acidifiedcalfskin material prepared in accordance with Example II was covered byspreading a thin polyethylene film into close contact with its exposedsurface. The sheet was then warmed to 37 C. slowly and held at thistemperature for 60 minutes. At the end of this heating a portion of thedissolved collagen content in the mixture had reconstituted as fibersand the sheet although tender could be handled without support. Thepolyethylene was stripped from the sheet and the sheet was immersed in achrome solution of the composition set forth in Example I for 60 minutesand was then removed and allowed to stand overnight.

The sheet was washed three times with tap water and solvent dried withacetone and it was found that the dried sheet had taken up about 3.0% ofchromic oxide based on the weight of the collagen material.

The chromium treated sheet was then subjected to furthe! treatmentcorresponding to the treatment of the chromium treated sheet of ExampleI.

The resultant product resembled a good grade of leather and was somewhatmore open than the product obtained according to Example 11.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

1. A method for forming skin-like collagen fiber masses "comprising thesteps of forming a mixture of existing collagen fibers and an aqueoussolution of collagen, said mixture being adjusted as to temperature andpH to prevent premature precipitation of collagen from solution shapingthe mixture into a sheet, thereafter bringing the sheet into intimatepenetrating contact with an aqueous solution containing an approximately30% to 50% basic chromium tanning compound, said solution of chromiumcompound containing from about 1% to about 5% by weight of chromiumcompound calculated as Cr O the pH and quantity of said solution ofchromium compound penetrating said sheet providing a pH in said sheet offrom about pH 3 to about pH 5, reacting said chromium compound with thedissolved collagen to precipitate collagen fibers containing at least0.2% by weight of combined chromium calculated as Cr O based on theweight of the precipitatedcollagen, and removing the fluid from thesheet.

2. A method for forming tanned skin-like collagen fiber massescomprising the steps of forming a mixture of existing collagen fibersand an aqueous solution of collagen, said mixture being adjusted as totemperature and pH to prevent premature precipitation of collagen fromsolution, shaping the mixture into a sheet, thereafter bringing thesheet into intimate penetrating contact with an aqueous solution havinga pH of fromabout 3 to about 4.5 and containing a masked approximately30% to 50% basic chromium tanning compound, said solution containingfrom about 1% to about 5% by weight of chromium compound calculated asCr O the pH and quantity of said solution of chromium compoundpenetrating said sheet providing a pH in said sheet of from about pH 3to about pH 5, reacting said chromium compound with the dissolvedcollagen and existing collagen fibers to precipitate collagen fibers andto provide a combined chromium content in said existing and precipitatedcollagen fibers of at least about 1% by weight calculated as Cr O basedon the weight of said collagen fibers, and removing the fluid from thesheet.

3. A method for forming tanned skin-like collagen fiber massescomprising the steps of forming a mixture of existing collagen fibersand an aqueous solution of collagen, said mixture being adjusted as totemperature and pH to prevent premature precipitation of collagen fromsolution, shaping the mixture into a sheet, hereafter bringing the sheetinto intimate penetrating contact with an aqueous solution having a pHof from about 3 to about 4.5 and containing a masked approximately 40%basic chromium tanning compound, said solution containing from about 1%to about 5% by weight of chromium corrpound calculated as Cr O andcontaining from about one to about three parts by weight of maskingagent selected from the group consisting of ammonium and alkali metalsalts of formic acid and hydroxy carboxylic acids to one part by weightof the chromium compound present in the solution, the pH and quantity ofsaid solution of chromium compound penetrating said sheet providing a pHin said sheet of from about pH 3.5 to about pH 5, reacting said chromiumcompound with the dissolved collagen and existing collagen fibers toprecipitate collagen fibers and to provide a combined chromium contentin said existing and precipitated col lagen fibers of at least about 1%by weight calculated as Cr O based on the weight of said collagenfibers, and removing the fluid from the sheet.

4. A method for forming tanned skin-like collagen fiber massescomprising the steps of forming a mixture of existing collagen fibersand an aqueous solution of collagen, said mixture being adjusted as totemperature and pH to prevent premature precipitation of collagen fromsolution, shaping the mixture into a sheet, thereafter bringing thesheet into intimate penetrating contact with an aqueous solution havinga pH of from about 3 to about 4.5 and containing a masked approximately40% basic chromium tanning compound, said solution containing from about1% to about 5% by weight of chromium compound calculated as Cr O andcontaining from about one to about three parts by weight ofsodiumformate as masking agent to one part by weight of the chromiumcompound present in the solution, the pH and quantity of said solutionof chromium compound penetrating said sheet providing a pH in said sheetof from about pH 3 to about pH 5, reacting said chromium compound withthe dissolved collagen and existing collagen fibers to precipitatecollagen fibers and to provide a combined chromium content in saidexisting and precipitated collagen fibers of at least about 1% by weightcalculated as Cr O based on the weight of said collagen fibers, andremoving the fluid from the sheet.

5. A method for forming tanned skin-line collagen fiber massescomprising the steps of forming a mixture of existing collagen fibersand an aqueous solution of collagen, adjusting the pH of the mixture tofrom about pH 3 to about pH 4.5, shaping the mixture into a sheet,thereafter bringing the sheet into intimate penetrating contact with anaqueous solution having a pH of from about 3 to about 4.5 and containinga masked approximately 40% basic chromium tanning compound, saidsolution con taining from about 1% to about 5% by weight of chromiumcompound calculated as Cr O and containing from about one to about threeparts by weight of sodium formate as masking agent to one part by Weightof the chromium compound present in the solution, the pH and quantity ofsaid solution of chromium compound penetrating said sheet providing a pHin said sheet of from about pH 3 to about pH 5, reacting said chromiumcompound with the dissolved collagen and existing collagen fibers toprecipitate collagen fibers and to provide a combined chromium contentin said existing and precipitated collagen fibers of at least about 1%by weight calculated as Cr O based on the weight of said collagenfibers, and removing the fluid from the sheet.

6. A method for forming tanned, skin-like collagen fiber massescomprising the steps of forming a mixture of existing collagen fibersand an aqueous solution of collagen and adjusting the mixture to a pH offrom about 5.2 to about 9.5 and ionic strength of 0.1 to 1.0 Whilemaintaining the temperature at not over about 5 C. to preventprecipitation of collagen from solution, shaping said mixture into asheet while maintaining the mixture at temperatures not over about 5 C.,thereafter bringing the layer into intimate penetrating contact with anaqueous solution having a pH of from about 3 to about 4.5 and containinga masked approximately 40% basic chromium tanning compound, saidsolution containing from about 1% to about 5% by weight of chromiumcompound calculated as Cr O and containing from about 1 to about 3 partsby weight of masking agent to 1 part by weight of the chromium compoundpresent in the solution, reacting said chromium compound With thedissolved collagen and lightly tanned fibers to precipitate collagenfibers and to provide a combined chromium content in said existing andprecipitated collagen fibers of at least about 1% by weight calculatedas Cr O based on the weight of said collagen fibers, and removing thefluid from the sheet.

7. A method for forming tanned skin-like collagen fiber massescomprising the steps of forming a mixture of existing collagen fibersand an aqueous solution of collagen at pH 5 to pH 9.5 and ionic strengthof 0.1 to 1.0 while maintaining the temperature at not over about 5 C.to prevent precipitation of collagen from solution, shaping the mixtureinto a sheet While maintaining the mixture at not over about 5 C.,thereafter raising the temperature of the mixture sufiicient partiallyto precipitate collagen fibers from dissolved collagen in the mixture,bringing the sheet into intimate penetrating contact with an aqueoussolution having a pH of from about 3 to about 4.5 and containing amasked approximately basic chromium tanning compound, said solutioncontaining from about 1% to about 5% by weight of chromium compoundcalculated as Cr O and containing from about one to about three parts byweight of masking agent to one part by weight of the chromium compoundpresent in the solution, the pH and quantity of said solution ofchromium compound penetrating said sheet providing a pH in said sheet offrom about pH 3 to about pH 5, reacting said chromium compound withremaining dissolved collagen and with existing collagen fibers toprecipitate such remaining dissolved collagen and to provide a combinedchromium content in said existing and precipitated collagen fibers of atleast about 1% by weight calculated as Cr O based on the weight of saidcollagen fibers, and removing the fluid from the sheet.

References Cited in the file of this patent UNITED STATES PATENTS

1. A METHOD FOR FORMING SKIN-LIKE COLLAGEN FIBER MASSES COMPRISING THESTEPS OF FORMING A MIXTURE OF EXISTING COLLAGEN FIBERS AND AN AQUEOUSSOLUTION OF COLLAGEN, SAID MIXTURE BEING ADJACENT AS TO TEMPERATURE ANDPH TO PREVENT PREMATURE PRECIPITATION OF COLLAGEN FROM SOLUTION SHAPINGTHE MIXTURE INTO A SHEET, THEREAFTER BRINGING THE SHEET INTO INTIMATEPENETRATING CONTACT WITH AN AQUEOUS SOLUTION CONTAINING AN APPROXIMATELY30% TO 50% BASIC CHROMIUM TANNING COMPOUND, SAID SOLUTION OF CHROMIUMCOMPOUND CONTAINING FROM ABOUT 1% TO ABOUT 5% BY WEIGHT OF CHROMIUMCOMPOUND CALCULATED AS CR2 O3, THE PH AND QUANTITY OF SAID SOLUTION OFCHROMIUM COMPOUND PENETRATING SAID SHEET PROVIDING A PH IN SAID SHEET OFFROM ABOUT PH 3 TO ABOUT PH 5, REACTING SAID CHROMIUM COMPOUND WITH THEDISSOLVED COLLAGEN TO PRECIPITATE COLLAGEN FIBERS CONTAINING AT LEAST0.2% BY WEIGHT OF COMBINED CHROMIUM CALCULATED AS CR2O3 BASED ON THEWEIGHT OF THE PRECIPITATED COLLAGEN, AND REMOVING THE FLUID FROM THESHEET.